Photovoltaic panels can be used for on-site, clean energy production. It is known in the art to connect modules in series to form module-level strings, which are in turn connected in parallel forming a photovoltaic (PV) array, e.g. such as used in commercial photovoltaic installations. A photovoltaic system thus uses many interconnected solar cells to convert sunlight into electricity. The photovoltaic system may comprise other components, such as mechanical and/or electrical connectors and mountings, voltage regulators, other means for modifying or modulating the electrical output, and tracking means for adjusting the orientation with respect to the sun. The array may for example be connected to an inverter, e.g. equipped with a Maximum Power Point Tracker (MPPT) to maximize the generated power. Even though such a PV system may, for example, operate under uniform lighting conditions, various conditions and/or varying conditions encountered in practice may lead to mismatch effects in the PV array.
Partial shading of the PV system can occur due to static objects, such as for example trees or chimneys, or due to dynamic shading conditions, e.g. due to clouds or soiling of the panel surface. Furthermore, incident shade can be caused by unpredictable events such as moving vehicles or objects, e.g. birds or leaves, and cloud covers. The direction, shape and density profile of such shadows is unpredictable, even though a temporal and spatial partial correlation may exist. It is known in the art that unpredictable shading events may temporarily cause strong differences between cells.
When the PV system is illuminated in a non-uniform manner, significant power losses are observed. This power loss is not necessarily linearly related to the casted shade, e.g. to the shadow area and/or shadow density. For example, the operation of cells which are not obscured, e.g. fully illuminated cells, can be affected by the shadow as well, for example due to a serial connection to another cell which is at least partially shaded. Furthermore, local hot-spots may threaten the proper operation of the system and may even cause permanent damage.
Several approaches can be used to deal with stochastic shading: (i) bypassing, (ii) global MPPT, (iii) decentralized MPPT, (iv) parallel solutions and (v) interconnection topology optimizations. Note that, ideally, in comparing the strengths of such techniques, other factors than power production should also be taken into account, such as installation cost and complexity.
In the bypassing approach, diodes are placed across groups of cells for isolation of the heavily shaded cells, such that the current bypasses them completely, e.g. bypass diodes may be placed over groups of cells to pass an excess current in order to prevent damage. Although this approach has its disadvantages, mismatch due to stochastic effects is handled in an elegant, but unfortunately also potentially suboptimal, manner. Under non-uniform operating conditions, the presence of bypass diodes affects the curve of the IV output of the array by creating local maxima. Depending on the previous operation point, e.g. the choice of operating voltage, an MPPT algorithm may not be able to detect efficiently the GMPP.
Global MPPT (GMPPT) refers to a class of techniques that aim to find the global maximum in the P-V curve of the array among multiple local maxima, e.g. in a centralized converter setup. Even though GMPPT can outperform traditional MPPT techniques, it still cannot harvest all available power because some cells are bypassed and/or a lower current is forced on the rest of the array. These losses can be shown to be non-negligible under typical dynamic shading conditions.
The third class of techniques mentioned above is decentralized MPPT. In such decentralized MPPT, each part of the system can operate at its local MPP. Different levels of control distribution can be used, such as MPPT at the level of an array, string, module, bypassed section or cell. Generally, a finer-grain MPPT can be advantageous for power harvesting.
The fourth class of solutions, referred to as parallel solutions hereinabove, provide a way to restore symmetry in a shaded panel by storing and then distributing energy to shaded cells. The power output increase from using battery-type solutions can be significant, but the control scheme that is used is complex, the extra hardware components imply an additional cost, and this type of approach does not provide easy reconfiguration.
The fifth class of approaches that can be used deal with stochastic shading by combining an even finer grained MPPT with improved interconnection topologies, either at the cell level inside the module or at the PV array level among panels. In such approaches, cells or panels are grouped together based on similar characteristics and controlled with dedicated MPPTs.
At the PV array level, a dynamic electrical array reconfiguration (EAR) strategy can be used that acts on the photovoltaic generator of a grid-connected PV system, based on a plant-oriented configuration, to improve the energy production when the operating conditions of the solar panels are different. A controllable switching matrix between the PV generator and the central inverter allows the electrical reconnection of the available PV modules. Furthermore, it is known that a change in the interconnections among the modules within a shaded PV field can impact its MPP. Several interconnection schemes have been favored based on the shaded pattern.
Where partial shading can be dealt with on a PV array level, as in some alternate approaches mentioned hereinabove, a per-panel reconfiguration of the cell interconnections might provide a finer-grain control and could be promising to extract an even higher amount of energy, e.g. close to the maximum possible energy, from the shaded cells.
The concept of having multiple converters per module, each connected to a group of similar solar cells, is introduced in US 2008/0135084. This arrangement enables efficient MPP tracking and maximum power harvesting. However, no details are disclosed on the implementation of the cell interconnection, except that both series and parallel connection schemes could be used for improved results.
In WO 2008/076301, an architecture for configuring cell interconnections is disclosed. In this architecture, a fully regular and uniform fine-grained grid-like interconnection network is arranged between the cells of a panel, allowing to interconnect cells in any sequence and any configuration, e.g. series or parallel. The configuration of the network can be performed at manufacturing time, or at in-the-field operation if switches and a controller are provided.
However, the configurations disclosed by the patent references cited hereinabove may entail a high implementation cost, due to the uniform and highly fine-grained application.
EP 2771753 discloses a module having a reconfigurable arrangement of photovoltaic cells, which are connectable to at least one DC-DC converter. The cells are non-divisibly joined in series, in substrings of at least two cells. The arrangement has an initial topology and a run-time topology, which are both non-uniform. The arrangement further comprises a means to reconfigure the arrangement at run-time by forming parallel and/or series and/or hybrid connections of the substrings, in which not all parallel or series connections are reachable by the reconfigurable arrangement. The reconfigurable arrangement is responsive to a non-uniform photonic stimulation by a non-uniform re-arrangement of the substrings. The cell-strings may comprise the columns or rows of the module, thus providing responsive adaptability of the module in one direction. Furthermore, a vertical split, e.g. in the direction corresponding to the orientation of the substrings, may be applied to allow some flexibility of the reconfiguration in two directions.
However, this bi-directional reconfiguration may have a cost associated therewith. For example, the vertical split may use switches inside the module, which can increase the cost significantly.